Piled Raft Slab

[rittz]

Can anyone help me with my first attempt at designing a piled raft foundation for a 30 ft dia. 60 ft high grain storage bin. The structure weighs 28,000 lbs and the product weight is 2,640,000 lbs.

We had designed a slab on grade (See fig 1). But our Geotech warns that frost heave could be a problem here in Canada. Insulating the slab is possible but difficult and not really a good option.

Does the pile configuration in Fig 2 make any sense?

We would like to ensure that all pimes hav close to the same load.. What order of slab thickness have you found works in a situations similar to this? Should we not spec a void form under the slab?

The available pile drilling equipment is set up for 24” dia. piles and can drill to 24 ft.

 

[rittz]

Attached is the pdf re referred to in my post. Sorry

 

[asixth]

I think you could look at removing some of the piles depending on the overturning.

The distribution of the loads to the piles thru the raft will be dependant of the slab stiffness, pile settlements and span lengths. For this configuration I think you would get an even distribution to the piles.

I remember reading a paper in the ACI a while back which recommends that a span to depth ratio less than 2 can be assummed rigid. In the days of FE packages you can assess this distribution thru an FE model.

 

[ishvaaag]

That I did whilst asixth posted ...

I contrarily even extended 20 inches the mat to get a more favourable distribution, I always endear regularity over economy. Just as a device to place the loads well in the FEM program I retained the rim at 20 inches thick whereas the 30 ft diameter central part I took to be 22 inches thick. Load is applied at service level and -with all criticable aspects of the model- the principal stresses at the bottom of the piled mat -that correspond to the tie part of one strut and tie model, atop are lower- show to be very moderate. Hence you may likely work at any reasonable thickness ... I probably would decide between 20 and 30 inches thick after examining lateral displacements and some tumbling irregularity etc.

 

[rittz]

Hi Aixth

Thanks so much for your reply. One question ..when you say "span" (in span to depth ratio ) do you mean the clear space bet ween piles?

****************

Thanks to ishvaag .. What do you mean by " "extended 20 inches the mat"?

Regards

 

[ishvaaag]

I mean that instead of having a 30 ft diameter mat I put 33.33 ft diameter mat; this was just to get a more proper distribution of the piles at 5 ft between centers, that asked for more piles but for a more regular fit at the perimeter. Anyway with proper analysis as asixth says you surely will find that you need not as much piles as I put or even you had in your scheme ... count the lateral forces to see, etc.